The present invention relates to a limit device for restricting the height of travel that a load can be lifted by a load lifting apparatus.
One typical load lifting apparatus is a dragline excavating machine used in open pit mines. This type of machine will be referred to as a dragline. A dragline generally comprises a mobile support that includes a boom 300 to 400 feet long having a sheave rotatably mounted adjacent its top end. A main lift rope of 3.5 to 4.5 inches in diameter is reeved over the sheave, to hang downward therefrom. A dragline bucket is connected to the main lift rope which is raised and lowered at speeds of over 800 feet per minute (when the bucket is not loaded) by a prime mover actuated by the dragline operator. The dragline bucket is frequently raised close to the top of the boom in order to spoil overburden high on a spoil pile or to reclaim a spoil pile.
The dragline operator must not over-lift the bucket beyond a predetermined lift position. The lift travel limit position is variable depending upon such factors as the length of and angle of the dragline boom and the overall vertical distance the bucket must be raised above the bottom of the mine pit. Raising the bucket beyond the lift travel limit position can cause the bucket to contact the boom structure, and this can result in serious damage to the dragline boom and bucket. As a practical matter, if the top end of the boom is 200 feet off the ground and the bucket is lifted at 800 feet per minute, it will take only 15 seconds for the bucket to reach the boom.
One normal procedure for minimizing the risk of such contact is to carefully train the dragline operator to observe when the load has reached the predetermined lift limit position. Draglines in open pit mines normally operate 24 hours a day in all weather conditions, and maximum risk occurs when the operator is forced to look into the sun, when the dragline is operating at night under artificial light, or when the dragline bucket must continually be raised to maximum height in dumping or reaching material from high on a hillside or spoil pile. In such operations, the bucket is repetitively raised at a high rate of speed and the effectiveness of training procedures depends upon visual acuity of the operator, physical response time, accurate visual estimates of distance, and visibility. Even if the operator thinks he has stopped the bucket in time, inertia forces can cause overtravel after control shut-off, in which case destructive contact can occur.